Detecting and handling heteroscedasticity in yield trial data

Abstract

Data from yield trials conducted in different environments are frequently analysed by ordinary analysis of variance. One of the preconditions for such analyses is that error variances be homogeneous across treatments and replicates. If a mixed model with fixed genotypes and random environments is assumed, genotype-environmental interaction is a random effect. For the ordinary ANOVA to be valid in this case, it is then also required that interaction variances be homogeneous for different genotypes. This paper suggests Box's correction of the ANOVA degrees of freedom for the case of heterogeneous interaction and error variances Different estimators of the correction factors are compared via Monte carlo Simulation.

Keywords:

• two-way layout
• randomized complete block design
• heteroscedasticity
• Box's correction
• ANOVA test
• interaction